The mechanisms behind the alteration of chemical carcinogenesis by the dietary lipotropes, choline, methionine, folic acid and vitamin B-12 have been studied. The metabolism and carcinogenic activity of ethionine in different species is being compared. The tissue levels of S-adenosylmethionine, S-adenosylhomocysteine, and 5-methylcytosine in animals treated with carcinogens, liver tumor promoters and methyl-deficient diets are being determined. The effects of dietary deprivation of methionine and choline on carcinogenesis in liver and other organs are under investigation using standard bioassays. The effects of methylase inhibitors on the methylation of macromolecules, on cell transformation in vitro and on carcinogenesis in vivo are determined.